Picking block length in a block bootstrap

I am using the Mann-Kendall test to assess trends in a data time-series. I believe there is autocorrelation in my data and therefore need to use a block bootstrap to correct for it. I have plotted the autocorrelation to try to determine the block size to use in my block bootstrap. I have not found many resources on how to select this block? Comparing one example to my data I thought I should pick a length of 15 as at lag =15 the points stay within the autocorrelation intervals (blue lines) but another source said 2-4 is usually a sufficient block length and that a block length of 1/4 of the sample size (n) can make the test insignificant. My data n=64 so 15 is approaching 1/4 of the sample size. How can I tell the best block size to pick from this plot (or another method?)

autocorrelation plot


Source : Link , Question Author : caitiekay , Answer Author : Community

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